In hard disk drives, data is written to and read from magnetic recording media, herein called disks, utilizing magnetoresistive (MR) transducers commonly referred to as MR heads.
Typically, one or more disks having a thin film of magnetic material coated thereon are rotatably mounted on a spindle. An MR head mounted on an actuator arm is positioned in close proximity to the disk surface to write data to and read data from the disk surface.
During operation of the disk drive, the actuator arm moves the MR head to the desired radial position on the surface of the rotating disk where the MR head electromagnetically writes data to the disk and senses magnetic field signal changes to read data from the disk.
Usually, the MR head is integrally mounted in a carrier or support referred to as a slider. The slider generally serves to mechanically support the MR head and any electrical connections between the MR head and the disk drive. The slider is aerodynamically shaped, which allows it to fly over and maintain a uniform distance from the surface of the rotating disk.
Typically, an MR head includes an MR read element to read recorded data from the disk and an inductive write element to write the data to the disk. The read element includes a thin layer of magnetoresistive sensor stripe sandwiched between two magnetic shields that are electrically connected together but are otherwise isolated. A constant current is passed through the sensor stripe, and the resistance of the magnetoresistive stripe varies in response to a previously recorded magnetic pattern on the disk. In this way, a corresponding varying voltage is detected across the sensor stripe. The magnetic shields help the sensor stripe to focus on a narrow region of the magnetic medium, hence improving the spatial resolution of the read head.
Earlier MR sensors operated on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varied as the square of the cosine of the angle between the magnetization and the direction of sense current flowing through the read element. In this manner, because the magnetic field of the recording media would effect the magnetization direction within the read element, the change in resistance could be monitored to determine the type of external magnetic field applied by the magnetic recording medium. Most current disk drive products utilize a different, more pronounced magnetoresistive effect known as the GMR or spin valve effect. This effect utilizes a layered magnetic sensor that also has a change in resistance based on the application of an external magnetic field. While multiple layers are typically used, the most relevant layers are a pair of ferromagnetic layers separated by an electrically conductive non-magnetic spacer layer such as copper. One of the ferromagnetic layers known as the “free” layer is a soft magnetic material whose magnetization is changed by the external magnetic field caused by the close proximity of the magnetic recording medium. The other ferromagnetic layer, known as the “pinned” layer, is also a soft magnetic material that has its magnetization direction fixed by an adjacent layer known as the “pinning” layer. A layer of antiferromagnetic material is typically used as the pinning layer. A sense current is passed from one end of the ferromagnetic and conductive layers to the opposite end of those same layers. The resistance of this tri-layer structure is proportional to the magnetization angle between the two ferromagnetic layers. Since one of the layers has a magnetization angle that is pinned and the other ferromagnetic layer has a magnetization that can vary in response to the magnetic field from an adjacent magnetic recording medium, the resistance of the tri-layer structure is proportional to that magnetic field from the recording medium. It has been discovered that this tri-layer structure behaves in this manner because of a spin dependent scattering of electrons, the scattering being dependent on the spin of the electron and the magnetization direction of the layer through which the electron passes.
As can be appreciated, the process of manufacturing GMR heads is very complex, involving the serial deposition of multiple layers on top of one another. At multiple times during the process of depositing the multiple layers, other processing steps such as etching away portions of a layer with a chemical, milling away portions of a layer by bombarding ions onto the layer, polishing a layer to optimize the surface thereof, and annealing the existing stack of layers by heating the same to a high temperature may be employed.
One undesirable characteristic of MR sensors is noisy response due to the buckling magnetic domain patterns in the sensor. This noise is often referred to as Barkhausen noise. This is typically minimized in MR sensors by the application of a small magnetic field longitudinally to the magnetic sensor that results in a single magnetic domain for the sensor. Barkhausen noise is typically suppressed in most current commercial products by hard biasing the MR sensor with permanent magnet regions that abut either end of the tri-layer sensor. Unfortunately, the abutted junction permanent magnet approach for longitudinal biasing does have its disadvantages. First of all, the magnetic field from the permanent magnet extends well past the boundary of the permanent magnet with the free layer. Because the magnetic field extends deep into the free layer, the sensitivity within the free layer to external magnetic fields from the magnetic recording medium is reduced. This reduced sensitivity is very undesirable in a GMR head. Second, stray fields from the permanent magnet tend to also be directed toward the shields on either side thereof, which also reduces sensitivity and increases noise. Third, the central area of the free layer tends not to be sufficiently biased. Lastly, the large granular structure within the permanent magnets makes precise and accurate stabilization with permanent magnets difficult.
Another type of longitudinal bias or stabilization that has been proposed is known as pattern exchange biasing (PEB). PEB involves creating the longitudinal bias only at two opposite ends of the tri-layer sensor structure. These biased-end sections provide only a small field to the remainder of the sensor, which is sufficient to largely prevent buckling domain patters so as to minimize Barkhausen noise while retaining the signal sensitivity of the central region of the sensor. This is accomplished by providing a layer of exchange material, such as an antiferromagnetic material like platinum manganese (PtMn). In this case, the principle of exchange results in the magnetic dipoles in the adjacent antiferromagnetic layer that are closest to the free layer causing the magnetic dipoles in the free layer to point in the same direction due to magnetic coupling. Because the free layer is ferromagnetic material, all of the magnetic dipoles in the layer will line up in the same direction as a result of this exchange biasing from the antiferromagnetic material (AFM). Because the central section of the magnetic sensor stripe is free of exchange material, the magnetization direction in that active region is free to rotate with the applied field from the adjacent magnetic recording media. GMR sensors using PEB for longitudinal stabilization have a much higher output, quieter response, and no stray fields, as compared to permanent magnet stabilized heads. Calculations show that the expected output increase of PEB over permanent magnet stabilization is approximately 60%–100%.
Most read head manufacturers desire to implement PEB and have been trying to do so, but have had trouble with the process complexity required by PEB. To date, few if any commercial disk drives include read/write heads with pattern exchange biasing of the GMR sensor. There are several reasons for this. First, it has been difficult to provide a clean and optimal surface on the free layer for depositing the patterned areas of antiferromagnetic material. When there is not a good surface, the exchange effect is significantly decreased. The typical manner for applying the patterned antiferromagnetic material is to first apply a column of photoresist material on top of a portion of the free layer. The remaining portions on top of the free layer, that will eventually have the antiferromagnetic material applied thereto, are then milled with argon ions to condition the surface of the free layer. The most critical area for the PEB is right at the boundary layer or edge of the antiferromagnetic material that will be deposited against the photoresist column. Unfortunately, because of the presence of the column, an effect known as “shadowing” can reduce the amount of ion milling of that portion of the surface and can also allow some of the milled material to collect in that area. When the AFM material is subsequently deposited, the exchange effect is decreased in the are next to the first photoresist material. Of course, the photoresist material is then lifted off. Since the free layer is not pinned well in this region, the resulting GMR sensor tends to pick up signals from adjacent tracks on the magnetic recording media. These side lobes tend to add noise to the signal from the GMR sensing element. In addition, the shadowing at the edge of the exchange layer produces a poor interface which leads to unstable performance and poor thermal stability.
In addition, there is an issue with the antiferromagnetic material that is used in the PEB technique. It should first be understood that a high temperature anneal process is required to set the direction for the pinning layer (also AFM material) to pin the pinned layer. This anneal process may be at 240° C. for six hours, for example. Because it is required that the magnetization direction of the pinned layer be different from (and preferably orthogonal to) the magnetization direction of the free layer, the annealing process for the pin layers is performed prior to application of the PEB AFM material. Subsequently, the antiferromagnetic materials are deposited and it is then necessary to anneal this material so as to set the magnetization direction of the free layer. Of course, since the entire stack is exposed to this second anneal process, it must not affect the magnetization direction in the pin layers or the device will not function. For this reason, there has been a fair amount of work in recent years with low blocking temperature AFM materials. Such materials can be annealed at a relatively lower temperature so that the AFM material can be set to a particular magnetization direction without affecting the pin layers. Unfortunately, these low blocking temperature AFM materials do not turn out to be stable in operational conditions. It should be understood that in operational conditions with current flowing through the sensor, the temperature of the sensor can become elevated to a point where the PEB AFM material may change its direction of magnetization so that the longitudinal biasing of the free layer no longer occurs. As this happens, the free layer will begin to break into the previously-described magnetic domains and the GMR sensor will have an undesirably high amount of Barkhausen noise.
A second technique for creating the PEB longitudinal stabilization is with the use of a reactive ion etch (RIE). With this process, the pin layers are deposited, followed by the free layer and then by a protective layer. An annealing process is then performed to set the pin layer. The protective layer is then removed with argon ion milling and an AFM material layer is deposited on top of the free layer, followed by a conductor layer on top thereof. The RIE process is then used to trench out the conductor and AFM material down to the free layer to expose a portion thereof A second anneal process can then be performed to set the PEB AFM material. Unfortunately, there are problems with this approach. When the protective layer is being removed with the argon milling, the plus or minus 5 angstrom resolution of the milling process and the 25 angstrom thickness of the free layer can result in either not removing enough of the protective layer (so that the interface between the free layer and the AFM PEB material is not clean) or removing too much of the free layer (which can effect the performance of the GMR's sensor).
As can be seen, there are many challenges that remain to be resolved before PEB longitudinal stabilization of GMR sensors is commercially feasible. It is against this background and a desire to improve on the prior art that the present invention has been developed.